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"And it is especially remarkable that, in the development of the posterior lobes, there is no approximation to the Lemurine, short hemisphered, brain, in those monkeys which are commonly supposed to approach this family in other respects, viz., the lower members of "the Platyrhine group.'

So far as the structure of the adult brain is concerned, then, the very considerable additions to our knowledge, which have been made by the researches of so many investigators, during the past ten years, fully justify the statement which I made in 1863. But it has been said that, admitting the similarity between the adult brains of man and apes, they are nevertheless, in reality, widely different, because they exhibit fundamental differences in the mode of their development. No one would be more ready than I to admit the force of this argument, if such fundamental differences of development really exist. But I deny that they do exist. On the contrary, there is a fundamental agreement in the development of the brain in men and apes.

Gratiolet or giuated the statement that there is a fundamental difference in the development of the brains of apes and that of manconsisting in this; that, in the apes, the sulci which first make their appearance are situated on the posterior region of the cerebral hemispheres, while, in the human foetus, the sulci first become visible on the fiontal lobes.76

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This general statement is based upon two observations, the one of a Gibbon almost ready to be born, in which the posterior gyri were "well developed," while those of the frontal lobes were" hardly indicated ' (1. c. p. 39), and the other of a human foetus at the 22nd or 23rd week of uterogestation, in wich Gratiolet notes that the insula was un. covered, but that nevertheless des incisures sement le lobe antérieur. "une scissure peu profonde indique la séparation du lobe occipital, tres

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Rolando, and one of the frontal sulci, plainly enough. Nevertheless, M. Alix, in his Notice sur les travaux anthropologiques de Gratiolet' (Mém. de la Société d'Anthropologie de Paris,' 1868, p. 32), writes thus: "Gratiolet a eu entre "les mains le cerveau d'un fœtus de "Gibbon, singe éminemment su"périeur, et tellement rapproché de "l'orang, que des naturalistes très

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corpétents l'ont rangé parmi les "anthropoïdes. M. Huxley, par ex"emple, n'hesite pas sur ce point. "Eh bien, c'est sur le cerveau d'un "fœtus de Gibbon que Gratiolet à "vu les circonvolutions du lobe tem"poro-sphenoidal déjà développées "lorsqu'il n'existent pas encore de plis 66 sur le lobe frontal. Il était donc "bien autorisé à dire que, chez "l'homme les circonvolutions appa "raissent d'a en w, tandis que chez "les singes elles se développent "d'w en a.

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"réduit, d'ailleurs dès cette époque. Le reste de la surface cérébrale "est encore absolument lisse."

Three views of this brain are given in Plate II. figs, 1, 2, 3, of the work cited, shewing the upper, lateral and inferior views of the hemispheres, but not the inner view. It is worthy of note that the figure y no means bears out Gratiolet's description, inasmuch as the fissure (anterotempo al) on the posterior half of the face of the hemisphere is more marked than any of those vaguely indicated in the anterior half. If the figure is correct it in no way justifies Gratiolet's conclusion: "Il y a donc entre ces cerveaux [those of a Callithrix and of a Gibbon] et "celui du foetus humain une difference fondamental. Chez celui-ci, longtemps avant que les plis temporaux apparaissent, les plis frontaux essayent d'exister."

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Since Gratiolet s time, however, the development of the gyri and su'ci of the brain has been made the subject of renewed investigation by Schmidt, Bischoff, Pansch,78 and more particularly by E ker,79 whose work is not only the latest, but by far the most complete, memoir on the subject.

The final results of their inquiries may be summed up as follows:1. In the human foetus, the sylvian tissure is formed in the course of the third month of uterogestation. In this, and in the fourth month, the cerebral hemispheres are smooth and rounded (with the exception of the sylvian depression), and they project backwards far beyond the cerebellum.

2. The sulci, properly so called, begin to appear in the interval between the end of the fourth and the beginning of the sixth month of foetal life, but Ecker is careful to point out that, not only the time, but the order, of their appearance is subject to considerable individual variation. In no case, however, are either the frontal or the temporal sulci the earliest.

The first which appears, in fact, lies on the inner face of the hemisphere (whence doubtless Gratiolet, who does not seem to have examined that face in his foetus, overlooked it), and is either the internal perpendicular (occipito-parietal), or the cal arine sulcus, these two being close together and eventually running into one another. As a rule the occipito-parietal is the earlier of the two.

3. At the latter part of this period, another sulcus, the "posterio, parietal," or " Fissure of Rolando" is developed, and it is followed, in the course of the sixth month, by the other principal sulci of the frontal, parietal, temporal and occipital lobes. There is, however, no clear evidence that one of these constantly appears before the other; and it is remarkable that, in the brain at the period described and figured by Ecker (1 c. p. 212–13, Taf. II. figs. 1, 2, 3, 4), the anterotemporal sulcus (scissure parallèle) so characteristic of the ape's brain, is as well, if not better developed than the fissure of Rolando, and is much more marked than the proper frontal sulci.

Taking the facts as they now stand, it appears to me that the order of the appearance of the sulci and gyri in the foetal human brain is in perfect harmony with the general doctrine of evolution, and with the

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view that man has been evolved from some ape-like form; though there can be no doubt that that form was, in many respects, different from any member of the Primates now living.

Von Baer taught us, half a century ago, that, in the course of their development, allied animals put on, at first, the characters of the greater groups to which they belong, and, by degrees, assume those which restrict them within the limits of their family, genus, and species; and he proved, at the same time, that no developmental stage of a higher animal is precisely similar to the adult condition of any lower animal. It is quite correct to say that a frog passes through the condition of a fish, luasmuch as at one period of its life the tadpole has all the cha1acters of a fish, and, if it went no further, would have to be grouped among fishes. But it is equally true that a tadpole is very different from any known fish.

In like manner, the brain of a human foetus, at the fifth month, may correctly be said to be, not only the brain of an ape, but that of a Arctopithecine or marmoset-like ape; for its hemispheres, with their great posterior lobster, and with no sulci but the sylvian and the calcarine, present the characteristics found only in the group of the Arctopithecine Primates. But it is equally true, as Gratiolet remarks, that, in its widely open sylvian fissure, it differs from the brain of any actual marmoset. No doubt it would be much more similar to the brain of an advanced fœtus of a marmoset. But we know nothing whatever of the development of the brain in the marmosets. In the Platyrhini proper, the only observation with which I am acquainted is due to Pansch, who found in the brain of a foetal Cebus Apella, in addition to the sylvian fissure and the deep calcarine fissure, only a very shallow anterotemporal fissure (scissure parallèle of Gratiolet.)

Now this fact, taken together with the circumstance that the anterotemporal sulcus is present in such Platyrhini as the Saimiri, which present mere traces of sulci on the anterior half of the exterior of the cerebral hemispheres, or none at all, undoubtedly, so far as it goes, affords fair evidence in favour of Gratiolet's hypothesis, that the posterior sulci appear before the anterior, in the brains of the Platyrhini. But, it by no means follows, that the rule which may hold good for the Platyrhini extends to the Catarhini. We have no information whatever respecting the development of the brain in the Cynnmorpha: and, as regards the Anthropomorpha, nothing but the account of the brain of the Gibbon, near birth, already referred to. At the present moment, there is not a shadow of evidence to shew that the sulci of a chimpanzee's, or orang's, brain do not appear in the same order as a man's.

Gratiolet opens his preface with the aphorism. "Il est dangereux "dans les sciences de conclure trop vite." I fear he must have forgo ten this sound maxim by the time he had reached the discussion of the differences between men and apes, in the body of his work. No doubt, the excellent author of one of the most remarkable contributions to the just understanding of the mammalian brain which has ever been made, would have been the first to admit the insufficiency of his data had he lived to profit by the advance of inquiry. The misfortune is that his conclusions have been employed by persons incompetent to appreciate their foundation, as arguments in favour of obscurantism.80

80 For example, M. l'Abbé Lecomte in his terrible pamphlet 'Le Dar

winisme et l'origine de l'Homme 1873.

But it is important to remark that, whether Gratiolet was right or wrong in his hypothesis respecting the relative order of appearance of the temporal and frontal sulci, the fact remains; that, before either temporal or frontal sulci, appear, the foetal brain of man presents characters which are found only in the lowest group of the Primates (leaving out the Lemurs); and that this is exactly what we should expect to be the case, if man has resulted from the gradual modifica tion of the same form as that from which the other Primates have aprung,

PART II.

SEXUAL SELECTION.

CHAPTER VIII.

PRINCIPLES OF SEXUAL SELECTION.

Secondary sexual characters-Sexual selection-Manner of action-Ex cess of males-Polygamy-The male alone generally modified through sexual selection-Eagerness of the male-Variability of the maleChoice exerted by the female-Sexual compared with natural selection -Inheritance, at corresponding periods of life, at corresponding seasons of the year, and as limited by sex-Relations between the several forms of inheritance-Causes why one sex and the young are not modified through sexual selection-Supplement on the proportional numbers of the two sexes throughout the animal kingdom-The proportion of the sexes in relation to natural selection.

WITH animals which have their sexes separated, the males necessarily differ from the females in their organs of reproduction; and these are the primary sexual characters. But the sexes often differ in what Hunter has called secondary sexual characters, which are not directly connected with the act of reproduction; for instance, the male possesses certain organs of sense or locomotion, of which the female is quite destitute, or has them more highly-developed, in order that he may readily find or reach her; or again the male has special organs of prehension for holding her securely. These latter organs, of infinitely diversified kinds, graduate into those which are commonly ranked as primary, and in some cases can hardly be distinguished from them; we see instances of this in the complex appendages at the apex of the abdomen in male insects. Unless indeed we confine the term "primary" to the reproductive glands, it is scarcely possible to decide which ought to be called primary and which Becondary.

The female often differs from the male in having organs for the nourishment or protection of her young, such as the mammary

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